Alarm switches and systems are known which permit signalling the theft or removal of an object from its place of rest or display. Examples include U.S. Pat. Nos. 4,274,088 to Pierson et al, 3,893,095 to DeJong et al, and 3,636,547 to Brace et al.
Conventionally, plunging trigger based alarm systems have incorporated a fixed switch contact and a movable switch contact, the throw length of which has been related to the length of the plunger movement required to set the alarm switch to the armed state. For concave underside display objects, as compared with a flat or shallow concave underside display object, a longer plunger length has been used to accommodate the extent of the concavity. When a flat-bottomed display object is to be protected with such a long plunger, a longer switch throw results, i.e., a longer plunger travel distance is required before contact is made and an alarm can be signalled. Flat-bottomed display objects do not require long plunger lengths; however, the shorter plungers used for flat-bottomed objects cannot accommodate concave underside display objects because they are too short.
In the Pierson et al device, a pair of switches are provided in which one switch extends upwardly and one switch extends downwardly. Thus, if the displayed object and the display riser base are separately or both moved together sufficiently to release either of the trigger switches, a built-in buzzer or siren can be sounded to signal a possible unauthorized removal or theft event. The pin or plunger trigger switch elements of Pierson et al require that the displayed object be flat-bottomed or that extra-length plunger elements be used with display objects having concave undersides, as is frequently the case. Thus, a selection of differing plunger lengths must be made available to accommodate both flat and concave underside objects of different concave dimensions, or individual units must be tailored or otherwise specially adapted to those objects having a concave underside.
The switches of Pierson et al do not automatically compensate for display objects with underside surfaces which vary from flat to greatly concave. A fixed position switch contact must be contacted by a movable switch contact traveling with the triggering plunger to complete the alarm circuit. When a long plunger (such as is needed for a concave underside display object) is used with a flat or shallow concave underside display object, the distance of plunger travel and thus the switch throw length, is dependent on the distance the plunger extends away from the fixed contact. When a switch having a long plunger is used with a flat or shallow underside display object, the throw of the switch can be very long. Certain other problems and limitations arise from the Pierson et al design, notably that the alarm can frequently be defeated by merely stopping release of the plunger before the alarm switch contacts are closed. That is, the displayed object can be lifted partially to expose the plunger, which can then be restrained while the display object is removed from the display site. This is especially true when one of the longer plungers intended for a deeply concave underside object is used with a flat underside object instead of the intended concave underside object.
In the DeJong device, a first spring-loaded trigger pin or plunger extends inwardly in a first channel to restrain movement of a second spring-loaded trigger switch pin or plunger which extends into another channel disposed normal to the first plunger. When the fire extinguisher is lifted, the first plunger is released and thus extends upwardly under spring pressure. As the first plunger is expelled past the end of the second plunger, the second plunger is released and the electrical circuit is completed to sound an alarm indicating removal of the fire extinguisher, thus signalling a possible fire condition. While not necessarily intended as an anti-theft device, the DeJong device alarm contact closure can readily be bypassed by restraining either the exposed first plunger or the second plunger while the fire extinguisher is removed, as with the Pierson et al device.
The Brace et al plunger is magnetically levitated against the display object underside. As with the Pierson et al and DeJong switches, full travel of the plunger is required before contact closure is made to sound an alarm. This is especially true when a longer plunger intended for a concave object is used with a flat underside display object. The Brace device requires a relatively strong magnet to levitate the plunger. Strong magnets, of course, cannot be used near items sensitive to magnetic fields, such as magnetically encoded stock tags, credit cards, and the like. The magnets of the Brace et al trigger device, being hidden from view, are particularly hazardous to magnetically encoded stock tags and credit cards, especially to stock tags because of the likely proximity of the tag and the undershelf magnet. In the case of any magnetically sensitive device, however, the hidden nature of the Brace et al switch may cause demagnetization of the encoded data without warning or knowledge. Loss of the encoded information is particularly inconvenient because it cannot be detected.
The switches disclosed in the Pierson et al, DeJong, and Brace et al patents are subject to being easily disabled or bypassed without indicating an alarm condition. That is, the length of movement occurring before contact closure to signal an alarm can be significant. This permits one to raise the displayed object slightly, or slide it to the side, and then depress and restrain the trigger switch without signalling an alarm. A strong planar member may be used to facilitate depressing and restraining the plungers of each of these devices.